1. Field of the Invention
The present invention relates to a variable output vane pump and, more particularly, to a variable output vane pump which is suitable for use with an automotive automatic transmission.
2. Description of the Prior Art
A hydraulic pump for an automatic transmission according to the prior art generally uses an inner or outer gearing of the constant output type. Since, in this case, an output proportional to the r.p.m. of the engine can be attained, it becomes more than necessary for a high r.p.m. so that most of it is returned from a flow regulator to an oil tank. The energy loss at this portion raises a problem. In order to solve this problem, it is necessary to make the pump of the variable output type so that the theoretical output of the pump itself may be reduced for an r.p.m. higher than a predetermined value.
The variable output pump thus far described is exemplified by a variable displacement pump, for example, which is disclosed in U.S. Pat. No. 4,342,545. This pump is an improvement over the prior art, but a displacement control chamber is formed between a ring and the inner wall of a housing so that seal portions are formed at two positions, i.e., between the ring and a pivot pin providing a pivot for the ring and between the ring and the pump housing. The sealing performance of the vane pump is so influenced by an error in the shape of the circular bore of the housing and by an error in the shape of the outer circumference of the ring that it can be maintained at a satisfactory level in some cases but with the sliding resistance being increased to raise a difficulty in the responsiveness to the output. In other cases, on the other hand, the responsiveness to the output is improved but with the sealing performance being degraded, thus raising an inconsistency. This inconsistency is followed by a disadvantage that the housing and the ring have to be machined with high accuracy.
There arises another disadvantage in that difficulty is encountered during machining because of the structure in which one half of the pivot pins for rocking the ring are fitted in the inner wall of the pump housing whereas the other half are fitted in semi-cylindrical grooves which are formed at the ring side. Moreover, since the suction and discharge of the pump are conducted in the axial direction through arcuate ports formed in the pump housing, there arises a defect in that the total length of the pump is enlarged in the axial direction. (In the case of a front-engine and front-drive automobile, it is necessary to make the axial length of the pump as small as possible.) On the other hand, the suction and discharge part arrangement is the so-called "one-side suction" which raises a problem in producing suction for a fast run. As to the sealing at the side opposite to the pivot of the ring, moreover, the center of the arc of the sealing face of the housing is offset from the center of rotation of the sealing portion, i.e., the center of the pivot. As a result, the spacing between the sealing portion of the ring and the sealing face of the housing is varied by the rocking motions of the ring which makes it difficult to ensure reliable sealing performance and durability so that the sliding resistance at the sealing portion is varied which raises the concern that the smooth sliding motions of the ring could be obstructed. Still moreover, the hydraulic force F.sub.R, which is generated in the ring by the pumping action exerted upon the ring, is applied as it is in the pivotting direction so that the contact load at the pivot portion becomes high thereby increasing the sliding resistance. There arises another problem that any excessive force F.sub.R blocks the smooth rocking motions of the ring.
According to the disclosure of U.S. Pat. No. 3,656,869, two separate opposed pressure chambers are formed between a cam ring and the bore of a housing by means of a sealing pin. Moreover, fluid circuitry including an automatic pressure regulator valve is in fluid communication with each of the pressure zones so that the pressure balance or ratio of pressures across the cam ring may be controlled. The structure thus disclosed succeeds in improving the problems of the sealing structure of the prior art described above. However, the seal pin is merely fitted loosely in the transverse openings of the outer circumference of the cam ring and may fail to ensure a complete sealing effect. Moreover, when the cam ring is controlled by the differential pressure between the two opposed chambers, there arises a concern that the maximum eccentricity of the cam ring cannot alway be held at the start of the pump so that the pump may not operate as expected.